Add massive neutrino support and improve CSV/CLI output

- Add neutrino mass parameter to SpectrumConfig and CLI interface
- Update PhaseSpace component to handle neutrino mass effects properly
- Update CSV headers to show basic endpoint and all transition types
- Add --csv-verbosity option to CLI with 'full' and 'concise' options
- Fix comparison plots to properly show linear scale variations
- Add comprehensive tests for neutrino mass functionality

beta_spectrum/components/phase_space.py

@@ -50,10 +50,26 @@ class PhaseSpace(SpectrumComponent):
         w_e = W
         p_e = momentum(W, self.m_e)
         w_nu = self.W0 - W
-        p_nu = momentum(w_nu, self.m_nu)
 
+        # Calculate p_nu, handling the case where w_nu < self.m_nu
+        # In this case, the neutrino energy is less than its mass, which is unphysical
+        # The phase space should be 0 in this region
+        if self.m_nu > 0:
+            # Check where w_nu < self.m_nu
+            below_threshold = w_nu < self.m_nu
+            # Calculate p_nu only where it's physically valid
+            p_nu = np.zeros_like(w_nu)
+            valid_mask = ~below_threshold
+            if np.any(valid_mask):
+                p_nu[valid_mask] = momentum(w_nu[valid_mask], self.m_nu)
+        else:
+            below_threshold = np.zeros_like(w_nu, dtype=bool)
+            p_nu = momentum(w_nu, self.m_nu)
+
+        # Calculate phase space factor
         phase_space = p_e * w_e * p_nu * w_nu
 
+        # Calculate forbid factor
         if self.tr_type in ["A", "F1"]:
             forbid_factor: float | np.ndarray = 1
         elif self.tr_type in ["F1U", "F2"]:
@@ -66,6 +82,11 @@ class PhaseSpace(SpectrumComponent):
             )
 
         result = np.asarray(phase_space * forbid_factor, dtype=np.float64)
+
+        # For regions below the neutrino mass threshold, the phase space should be 0
+        if self.m_nu > 0:
+            result[below_threshold] = 0.0
+
         if self._logger:
             self._logger.debug(
                 "PhaseSpace: output range=[%.6e, %.6e]",